The present invention relates to an apparatus for measuring the relative position of two parts, having a deformable coil which is connected to both parts and deforms according to their relative position, with the inductance of the coil being form-dependent and thus being a measure of the relative position.
Apparatuses of this type are known from DE 11 08 449, DE 32 05 705, DE 41 41 545 or DE 199 13 869. The inductance of the coil is used, for example, as frequency-determining part of a resonant circuit, the frequency of which is measured and is representative of the relative position of the parts; relative movements of the parts can be ascertained subsequently from the temporal change of the relative position.
In all known apparatuses, the coil is formed by one or two concentric or parallel helical springs which constitute simple straight air-core coils. Such air-core coils have the characteristic that their magnetic lines of force close outside the air-core coil. In other words, the magnetic lines of force exit the ends of the helical springs and form a stray field in the surrounding area. In the known embodiments with two concentric or parallel helical springs as well, both helical springs act as open air-core coils with a corresponding stray field.
The occurrence of this stray field has very considerable effects on the proportionality between form change and inductance change of the coil. By way of example, the inductance L of a cylinder-helical coil is calculated at
                    L        =                              k            ·                          μ              r                        ·                          μ              0                                ⁢                                    A              ·                              N                2                                      l                                              (        1        )            where                k . . . demagnetization factor        μr . . . relative permeability        μ0 . . . permeability        A . . . cross-sectional area of the winding        N . . . number of turns of the winding        L . . . length of the winding        
The inductance L is thus substantially indirectly proportional to the length l of the winding, e.g. the current stretching or compression of a tension or compression spring used as a coil. The stray field proportion of the coil can be expressed here in terms of the demagnetization factor k which is influenced, to a degree which can only be calculated with difficulty, by the coupling in of external electromagnetic influences, by the permeability of the medium (air) outside the coil and, not least, by assemblies in the surrounding area of the coil. Furthermore, the occurrence of the stray field also means the emission of disturbing electromagnetic waves into the surrounding area.
The object of the invention is to overcome the disadvantages of the known constructions and to provide a measurement apparatus, which is not susceptible to disturbances, with improved measurement accuracy and reproducibility, while having as simple a design as possible.